1. Field of the Invention
The present invention relates to an improvement in a method for preparing a superconducting thin film of compound oxide having a smooth surface by laser evaporation technique.
2. Description of the Related Art
Superconductivity has been thought, for a long time, to be a phenomenon observable at ultra-low temperatures realized with liquid helium as a cryogen. However, the possibility of an existence of a new type of superconducting material was revealed by Bednorz and Muller, who discovered (La, Sr).sub.2 CuO.sub.4 which exhibits the superconductivity at 30 K in 1986. C. W. Chu et al. discovered, in the United States of America, another superconducting material of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. having the critical temperature of about 90K in 1987. Maeda et al discovered so-called bismuth type superconducting material. And hence, the possibility of an actual utilization of high-Tc superconductors has burst onto the scene because the superconductivity can be realized with cheap cryogen of liquid nitrogen.
The new type superconducting materials were obtained in a form of sintered mass prepared by powder metallurgy but now they can be prepared in a thin film form of very high quality.
Laser evaporation technique is one of the most promising techniques. In fact, superconducting thin films which exhibit improved superconducting properties can be prepared by the laser evaporation technique with no after-treatment such as annealing of thin films deposited on a substrate.
It is true that a superconducting thin film of the oxide of high quality can be prepared by the laser evaporation technique but the laser evaporation technique has the disadvantage that undesirable fine particles of oxide are produced on a surface the thin film obtained. Existence of such fine particles is undesirable for fabricating superconducting devices in which smoothness of the superconducting thin film is requested.
Therefore, an object of the present invention is to solve the problem and to provide a novel process for preparing a superconducting thin film of oxide possessing an improved surface property by the laser evaporation technique.